A gating grid, which is particularly used in IMS spectrometers, is, for example, known from U.S. Pat. No. 4,150,319. This known gating grid comprises two sets of parallel wires, each set consisting of a single, continuous wire which is strung back and forth across an insulating frame similar to the stringing of a tennis racquet. In this way, the wires of the two sets are fed through holes in the frame so that the parallel wire parts of the two sets alternate. When a voltage is applied between the two wires, electric fields with field lines between the alternately-biased wires are formed in the plane of the grid. These electric fields alternately change their direction, but are essentially directed perpendicular to the normal vector of the grid plane, which coincides with the direction of the ion current to be gated.
When compared to older types of gating grids, where the two wire sets were arranged in closely-spaced, but nevertheless different parallel planes (so-called Bradbury-Nielsen grids), this known grid has the advantage that almost no components of the electric field are present in the direction of the ion current (the axial direction of an IMS spectrometer). Therefore, the gating performance is considerably improved. In particular, when the grid is closed, the residual current, which in spectrometric use leads to a background signal, is noticeably reduced. The switching performance is also improved, i.e. when the grid is switched off, the current drops faster to zero.
However, this known gating grid has the disadvantage that it can only be produced in a costly way and with limited reproducibility. Moreover, for finite wire diameter, the gating plane is not precisely defined, leading to a broadening of the switching processes.
Therefore it, is an object of the invention to provide a gating grid of the kind mentioned above where the switching performance is improved.
It is another object of the invention to provide a method for the production of the improved grid in such a way that the gating grid can be produced in a simple, reproducible and reliable manner.
It is a further object to provide an improved IMS spectrometer which uses such a gating grid to achieve improved sensitivity, resolution and reliability.